The goal of this project is to identify factors that regulate neurotransmitter release kinetics. The motivation for this project is two-fold. First, slowed release of synaptic vesicles (SVs) prolongs post-synaptic currents, which can change post-synaptic excitability and action potential firing patterns. Thus, release kinetics has profoun effects on circuit development and cognition. Second, mutations linked to Autism alter post-synaptic response kinetics by various mechanisms. The SVs available for release comprise multiple functionally distinct pools, which fuse with different kinetics and release probabilities. Synchronous (or fast) release occurs over a few milliseconds while delayed (or slow) release occurs over 10-100 ms. The detailed mechanisms regulating release kinetics have not been determined. We propose to identify factors and mechanisms that tune release kinetics, using C. elegans as a model system. In preliminary studies, we identified three syntaxin-binding proteins that dictate release kinetics at neuromuscular junctions (NMJs). We showed that UNC-13L promotes fast release, UNC-13S promotes slow release, and Tomosyn (a third syntaxin-binding protein) inhibits slow release. Based on these preliminary results, we will determine how UNC-13L accelerates release, how the different UNC-13 proteins produce differences in spontaneous release, and how SVs are coupled to calcium channels. These studies should provide new insights into the biochemical mechanisms regulating release kinetics.